<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.14/vue.js"></script>
</head>
<body>
  <div id="root">

  </div>
  <script>
    // mixin
    const empty_mixin = Vue.mixin({
      data () {
        return {
          name: ''
        }
      },
      filters: {
        empty: (value) =>{
          return value || '--'
        }
      }
    })
    new Vue({
      el:'#root',
      mixins:[empty_mixin],
      data:{
        user: 'world'
      },
      template: `
        <h1>hello , {{ user | empty }}</h1>
      `
    })
  </script>
</body>
</html>